2 Questions:
1) Did you use alternatives --config mta to select postfix as your mta?
2) Does editing the master.cf file to use just "rmail" work?
After selecting postfix as your mta rmail should be a link to rmail.postfix.

I can answer both questions with yes.
The point is that without the change I described postfix will put
"rmail.postfix" as the command to execute on the remote machine. Even if the
remote machine would have rmail.postfix, it doesn't work because "rmail.postfix"
most likely isn't in the list of commands that the UUCP system _may_ execute.